Sensed switches are often used to control the operation of loads instead of powered switches. Powered switches are disposed serially with a load to directly control current delivered to the load. In contrast, sensed switches control the load current indirectly. The state of the switch, open or closed, is instead sensed by applying a low current signal and measuring the voltage. The opportunity to use a low current, voltage measurement leads to reduced wiring harness complexity, weight, and costs. In complex electrical systems with numerous switch-controlled loads, such as automobile vehicles, the cost savings may be considerable.
Determining the state of a sensed switch typically involves applying a test current and a voltage comparison. For example, a voltage level dictated by the state of the switch is compared with a threshold voltage. The voltage level is ideally not dependent on the voltage drop across the switch contacts. But unfortunately, the switch contacts oxidize over time due to humidity and contamination, increasing the resistance presented by the switch itself. The increased resistance and test current result in an increase in the sensed voltage, thereby increasing the risk of incorrect operation (i.e., detection). Switch contact oxidation may be especially challenging in connection with normally open switches, i.e., switches with contacts that close upon application of an external force.
The oxidation challenge presented by sensed switches is not applicable to the powered switch approach. In powered switches, the current levels are high enough to burn off any oxidation of the switch contacts. Because the current levels may be much lower with sensed switches, a wetting current is used to remove the oxidation from the switch contacts. The wetting current is typically a temporary current level of the current that flows through the switch contacts when the switch transitions from open to closed. The temporary current level is sufficient to remove the oxidation. A circuit used to detect the state of the switch may also be configured to control the application of the wetting current.